It's not that easy unfortunately
I recently spent weeks with a test install of Ubuntu 12.10 that I used as a substrate to install : Lubuntu (LXDE), Xubuntu (Xfce), Mate (GTK2), Cinnamon (GTK3) & KDE (Qt4) in addtion to trying again Unity.
Yes you can install any of these using the package manager built in to Ubuntu but in every case there were problems, bugs, missing bit's. Mint provides these choices in a really polished fully complete package.
It's doing a disservice to Linux to pretend that it is sooooo easy to just throw together anything. It's not. It takes serious man hours of high level developer time to produce quality distro's. Don't be pretending otherwise.
I'm not 13 years old or a retiree with unlimited tinckering time.
I need a distro that's polished, complete & easy to use. That used to be Ubuntu until the onset of Unity disease. It is'nt any more unfortunately.
Rather than knee jerk I devoted one full week of using Gnome3 desktop and then one full week using Unity just to make sure I was'nt being to quick to judge. Gnome3 I can put up with if I had no other choice but the ergonomics are terrible. Unity? it's _worse_ than Gnome3. It's just a half assed mess that is painfuly bad.
Little things that drive new users nuts?
Persistence of settings eg I want my sound levels & screen brighness to remain at the same level I left them at last time I logged out.
The only two (usable by mere mortals) desktops that do are Mate (GTK2) & KDE (Qt4).
